Re: [PATCH 0/5] Add process_memwatch syscall

[Muhammad Usama Anjum]

On 8/10/22 2:22 PM, Peter.Enderborg@xxxxxxxx wrote:
> On 7/26/22 18:18, Muhammad Usama Anjum wrote:
>> Hello,
>>
>> This patch series implements a new syscall, process_memwatch. Currently,
>> only the support to watch soft-dirty PTE bit is added. This syscall is
>> generic to watch the memory of the process. There is enough room to add
>> more operations like this to watch memory in the future.
>>
>> Soft-dirty PTE bit of the memory pages can be viewed by using pagemap
>> procfs file. The soft-dirty PTE bit for the memory in a process can be
>> cleared by writing to the clear_refs file. This series adds features that
>> weren't possible through the Proc FS interface.
>> - There is no atomic get soft-dirty PTE bit status and clear operation
>>   possible.
>> - The soft-dirty PTE bit of only a part of memory cannot be cleared.
>>
>> Historically, soft-dirty PTE bit tracking has been used in the CRIU
>> project. The Proc FS interface is enough for that as I think the process
>> is frozen. We have the use case where we need to track the soft-dirty
>> PTE bit for running processes. We need this tracking and clear mechanism
>> of a region of memory while the process is running to emulate the
>> getWriteWatch() syscall of Windows. This syscall is used by games to keep
>> track of dirty pages and keep processing only the dirty pages. This
>> syscall can be used by the CRIU project and other applications which
>> require soft-dirty PTE bit information.
>>
>> As in the current kernel there is no way to clear a part of memory (instead
>> of clearing the Soft-Dirty bits for the entire processi) and get+clear
>> operation cannot be performed atomically, there are other methods to mimic
>> this information entirely in userspace with poor performance:
>> - The mprotect syscall and SIGSEGV handler for bookkeeping
>> - The userfaultfd syscall with the handler for bookkeeping
>>
>>         long process_memwatch(int pidfd, unsigned long start, int len,
>>                               unsigned int flags, void *vec, int vec_len);
>>
>> This syscall can be used by the CRIU project and other applications which
>> require soft-dirty PTE bit information. The following operations are
>> supported in this syscall:
>> - Get the pages that are soft-dirty.
>> - Clear the pages which are soft-dirty.
>> - The optional flag to ignore the VM_SOFTDIRTY and only track per page
>> soft-dirty PTE bit
>>
> 
> Why can it not be done as a IOCTL?
It can be done as ioctl. I think this syscall can be used in future for
adding other operations like soft-dirty. This is why syscall has been added.

-- 
Muhammad Usama Anjum